This proposal aims to advance our understanding of the molecular mechanisms that regulate the development of the vestibular system by examining the biochemical function of a novel gene Otopetrin1 (Otop1) that is required for normal otoconial development in the mouse. The mouse mutants tilted (tlt) and mergulhador (mlh) both contain distinct missense mutations in Otop1. These mouse mutants specifically lack otoconia but have an apparently normal sensory epithelium. This specific defect suggests that Otop1 protein may be involved in the early steps of otoconial formation, in particular, the nucleation and growth of calcium carbonate crystals around an organic core. In this proposal, I will examine the topology, orientation, and biochemical function of Otop1 in vitro and in vivo and will determine how the tlt and mlh missense mutations affect Otop1 protein structure or function. [unreadable] [unreadable]